The present invention relates to an optical magnetic recording medium for carrying out a recording, a reproducing and an erasing of information, and a method of reproducing information in an optical magnetic recording medium.
A reproduction of a record on an optical magnetic disk is carried out by using a laser beam. A spot diameter on a recording film can be expressed by .lambda./NA, where NA represents the numerical aperture (usually about 0.5) of an objective lens to be used and .lambda. represents a wavelength of a laser. When the wave-length is 680 nm and NA is equal to 0.55, the spot diameter becomes about 1.2 .mu.m. An existence of only one record magnetic domain (mark) in the spot diameter is a proper condition of reproduction. In other words, the shortest length of the record magnetic domain that can be reproduced is 1.2 .mu.m/=0.6 .mu.m.
As described above, the recording density is improved when the wavelength of a semiconductor laser is short. However, under current circumstances, it is not possible to expect the shortening of the wavelength of a semiconductor laser at a very early stage. Relating to this, a technique for improving a reproduction resolution by utilizing the magnetic characteristics of a recording film has been proposed in Japanese Patent laid-open Publication No. JP-A-3-93056, for example.
According to this technique, a recording film is structured by three kinds of magnetic film, and a part of information recorded on another magnetic layer is compulsively masked by a magnetic layer at a beam incident side to effectively improve the reproduction resolution. The recording film is structured by the three kinds of magnetic layer of a reproduction layer, an intermediate layer and a recording layer, from the beam incident side.
The above-described content is a magnetic super-resolution effect and this is called "magnetic super-resolution". A magnetic mask shape to be compulsively generated within a spot is determined by a temperature distribution on a recording film generated by a reproduction light.
There are the following three main types in the above-described magnetic super-resolution:
A first is FAD (Front Aperture Detection) as disclosed in the Japanese Patent laid-open Publication No. JP-A-3-93056 according to which a magnetic domain can be observed only at a low temperature section within a light spot. A second is RAD (Rear Aperture Detection, reference the Japanese Patent laid-open Publication No. JP-A-63-93058) according to which a magnetic domain can be observed only at a high temperature section within a light spot. A third is CAD (Center Aperture Detection, reference the Japanese Patent laid-open Publication No. JP-A-5-12731).
The first (FAD) is effective for improving the scanning density but it is not possible to increase the track density because a magnetic domain of a low temperature section, that is, the magnetic domain recorded on both sides of the adjacent tracks, is also leaked into a signal. Further, the FAD also has a handicap of an apparatus that a reproduction magnetic domain of several hundred Oe is necessary.
In the case of the second (RAD), only the information at a high temperature section, that is, the information near the center of the spot, can be observed. Therefore, there is no leakage of a magnetic domain of the surrounding low temperature section into a signal. Accordingly, the RAD is suitable for increasing both the track density and the scanning density. However, a magnetic domain copied to a reproduction layer deviated from the spot remains as it is, and therefore the magnetic super-resolution effect cannot be exhibited at the time of reading the information next time.
Accordingly, in the case of the RAD, it is necessary to erase in advance the magnetic domain copied to the reproduction layer at a sufficiently low temperature of the disk at the reproduction section. Thus, in the case of the RAD, it is also necessary to install a large permanent magnet of several kOe within the apparatus. This is a problem in providing a compact apparatus.
A magnetic field which is sufficiently large enough for preparing the magnetization of the reproduction layer in one direction is called an "initialization magnetic field" which is the magnetic field necessary for the initialization. This can be given by the following expression.
A minimum required initialization magnetic field=(coercive force of the reproduction layer)+(exchange-coupled magnetic field which is received by the reproduction layer from the adjacent magnetic layers)+(magneto-statically coupled magnetic field which is received by the reproduction layer from the adjacent magnetic layers) - - - (1)
In this case, the exchange-coupled magnetic field is a magnetic field which tries to cancel the torsion of a spin between the reproduction layer and the adjacent magnetic layers. The magneto-statically coupled magnetic field in this case refers to a portion of the magnetic field that extends from the inside of a magnetic domain recorded on a recording layer to the outside and affects the reproduction layer. Accordingly, in order to reduce the initialization magnetic field, it is necessary to make smaller the exchange-coupled magnetic field or the magneto-statically coupled magnetic field that is received by the reproduction layer from another magnetic layer. However, when the initialization magnetic field is reduced by making smaller the exchange-coupled magnetic field or the magneto-statically coupled magnetic field at ambient temperature, the magneto-statically coupled magnetic field and the exchange-coupled magnetic field are made further smaller at a high temperature, so that the information on the recording layer cannot be copied to the reproduction layer. Consequently, there is a limit to the reduction of the initialization magnetic field.
The third (CAD) does not require the initialization magnetic field. In the CAD, a magnetic layer having such characteristics that the magnetization faces within the plane at ambient temperature but faces a perpendicular direction when the temperature rises is used for the reproduction layer. In the case of the above-described RAD, there are only two types of magnetization status of the reproduction layer that the magnetization is either in the upward direction or in the downward direction and it is possible to achieve a contrast necessary for a super-resolution. However, in the case of the CAD, a super-resolution effect as obtained in the RAD cannot be achieved because there are various statuses of magnetization of the reproduction layer, ranging from the upward direction to the downward direction.
As explained above, each of the FAD, RAD and CAD has problems in practical application. In other words, according to the above-described prior-art techniques, an optical magnetic recording medium which can carry out a super-resolution reproduction at high contrast with both high track density and high scanning density and which does not require a reproduction magnetic field and an initialization magnetic field has not been obtained.
With a view to eliminating the above-described drawbacks, it is an object of the present invention to provide an optical magnetic recording medium which can satisfy all the above-described requirements, and a recording and reproducing method using the same. More particularly, it is an object of the present invention to provide an optical magnetic recording medium which does not require either an initialization magnetic field or a reproduction magnetic field and which can carry out a high-density reproduction with a magnetic super-resolution effect at 200 Oe or below that does not affect the structure of an apparatus, and a recording and reproducing method using the same.
In the abstract, the present invention includes a recording layer for holding information, a reproduction layer to which information on a recording film is copied and an intermediate layer positioned between the recording layer and the reproduction layer, wherein the intermediate layer has an in-plane magnetization component within a specific temperature range.
It is desirable that the recording layer has an easy magnetization axis perpendicular to the surface of the medium and holds information in either the upward or downward magnetization status. It is also desirable that the reproduction layer has an easy magnetization axis perpendicular to the surface of the medium and the information on the recording film is copied to this reproduction layer. It is further desirable that the recording layer and the reproduction layer are calibrated as vertical magnetization films. The intermediate layer has an in-plane magnetization component within a specific temperature range and plays the role of a switching layer by losing the magnetization component within a specific temperature range.
Further, the present invention includes a recording layer for holding information in the status of either an upward magnetization or a downward magnetization, a reproduction layer closely adjacent to a recording film and an intermediate layer located between the recording layer and the reproduction layer. Further, the present invention uses an optical magnetic recording medium in which the intermediate layer has an in-plane magnetization component within a first temperature range and loses the in-plane magnetization component within a second temperature range, and at the time of reproducing information, maintains the intermediate layer locally within the second temperature range by an irradiation of an energy beam so that the information on the recording layer is copied to the reproduction layer to enable observation of the copied information. Alternately, an in-plane magnetization film is used for the intermediate layer, and has a characteristic that the magnetic characteristics are switched between the first temperature range and the second temperature range. Normally, at the time of information reproduction, the in-plane magnetization film is locally heated by the irradiation of an energy beam so that the information on the recording layer is copied to the reproduction layer to enable observation of the copied information.
As a detailed structure of a medium, an optical magnetic recording medium is provided having a dielectric film formed on a substrate, a recording film including at least three layers of magnetic film formed by either a magneto-static coupling or an exchange-coupling on said dielectric film, and a recording film formed by a dielectric film and/or a metal film. Further, in between the reproduction layer and the recording layer of the above-described recording films, an intermediate layer in which a magnetization is faced in-plane within a temperature range from at least 50.degree. C. to the Curie temperature is inserted. Further, it is desirable that the sum of the coercive force of the reproduction layer and the magneto-statically coupled magnetic field and the exchange-coupled magnetic field received by the reproduction layer is smaller than 200.
This application relates to U.S. patent application Ser. No. 08/126,766 filed on Sep. 27, 1993 entitled Overwritable Optical Recording Medium and Recording Method of the Same, by Takeshi MAEDA et al. and assigned to HITACHI, LTD.